[RE-wrenches] P1 micro performance

[Exeltech]

No.  There's no increase in the waveform distortion when an inverter output
is being limited to some maximum output wattage.

(That's the exact reason *not* to call it "clipping".  Clipping IS
distorted / distortion / harmonics.)

To ensure a grid-tied inverter IS operating at its maximum, the DC power
supplies used as the power source are rated much more than the inverter
wattage.

As part of the UL 1741 certification process, inverters must be operated
at 100% of their ratings (among other power levels.)  Harmonics are
measured and quantified.  If at any time the inverter exceeds the harmonic
limit allowed, it fails UL 1741.


Dan

--- On Fri, 3/22/13, William Dorsett <wmdorsett at sbcglobal.net> wrote:

From: William Dorsett <wmdorsett at sbcglobal.net>
Subject: Re: [RE-wrenches] P1 micro performance
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Friday, March 22, 2013, 9:52 AM

OK, if the upper limit if the curve is “flat topped” do we get increased problems with harmonic noise at the knee as you would in modified “square” wave?  Bill DorsettManhattan, KS  From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of David Brearley
Sent: Friday, March 22, 2013 8:43 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] P1 micro performance  Thanks for the clarification.   FWIW: Flat topping is exactly what occurs. Inverter limiting clips the inverter output power curve (not the voltage or current wave forms). The chart below has one data point for every hour of the year. The clipped/flat-top area is the result of the 225 kW inverter limiting the power output of a 385 kW array:       On Mar 22, 2013, at 1:07 AM, boB wrote:

On 3/21/2013 9:59 PM, Exeltech wrote:Wrenches,

I'm probably a lone voice on this .. and not intending to get overly picky.

No, two lonely voices, Dan.

I associate clipping with audio waveforms which stops
the negative or positive voltage peaks flat.  Also called
flat-topping.

Limiting is like turning down the volume.  The waveform
stays the same and does not distort as it would if it
were being flat topped (and flat bottomed)

Thanks !
boB






Could we call power limiting what it is .. "limiting", and not "clipping"?

Clipping implies distortion, which isn't the case here.  Limiting is just that.
The inverter output is limited to some maximum value -- not "clipped".

The output power curve flattens when integrated over time, but this still isn't
distortion in the waveform.  It's simply a point in the output where the derivative
is zero.  Not increasing, not decreasing.  Just .. zero.  No additional increase
in the output for an increase in available energy at the input.  Think "governor"
on an engine....

Thanks.


Dan Lepinski, Sr. Engineer
Exeltech / Exeltech Solar Products


--- On Thu, 3/21/13, David Brearley <david.brearley at solarprofessional.com> wrote:


From: David Brearley <david.brearley at solarprofessional.com>
Subject: Re: [RE-wrenches] P1 micro performance
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Thursday, March 21, 2013, 11:37 PMThanks for sharing the screen capture, Marco.  Interesting issues to think about here. This is actually prime clipping season in many places (not sure about Hawaii) due to the cool weather. While there are more sun-hours in the summer, the cell temperatures are often high enough that you won't tend to see rated power out of the modules.   While I'm not running performance models for work, the people who do are routinely increasing dc-to-ac ratios, often as high as 1.4-to-1. Having said that, most inverters aren't installed on a roof. (Not yet anyway.)   I'd probably lean to a more conservative sizing ratio for micros. While I can imagine some scenarios where I'd be comfortable with a 215 W micro on a 265 W module—like a flat roof install in Vermont, which reportedly doesn't see 1,000 W/m^2 very often—I wouldn't try that here in Texas.

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